Selective signalling



May 11, 1948.

H.- S. JONES SELECTIVE SIGNALLING Original Filed Nov. 4, 1942 INVENTOR. HARRY S. JONES.

ATTORNEY at... May n, 1948 UNITED. STATES 7 PATENT OFFICE,

' SELECTIVE SIGNALLING Harry S. Jones, East Orange, N. 3., asslgnor to The Brown Instrument Co mpany, Philadelphia,

q Pa., a corporation of Pennsylvania application November 4 1942, Serial No. 464,586. Divided and this application January 10, 1946, Serial No. 640,285

5 Claims.

from normal, or may consist of or actuate adlusting means tending to restore the condition under measurement to its normal value on variations therefrom, or may consist of or actuate both indicating or adjusting means.

The present invention was primarily devised for use in indicating and controlling furnace conditions in installations in which the furnace temperature is measured by means of a thermoelectric element, such as a thermocouple. and an instrument responsive to the variations in electromctive force produced by the thermocouple.

- It will be understood, however, that the present invention is not limited to such use and also has especial utility in measuring the magnitude of electrical, chemical, physical, and mechanical quantities and qualities such as electric current, hydrogen ion content, pressure or flow.

In one embodiment of my invention the instrument for measuring the thermocouple electromotive force has associated therewith a selective.

relay which is actuated in one way when the electromotive force of the thermocouple falls below a predetermined normal value and in a different way when the electromotive force rises above the normal value, and in a third way when the thermocouple electromotive force is the normal value. The selective relay is employed to operate furnace control elements which may comprise signalling devices from the actuation of which the furnace may be directly controlled as by the manipulation of manual control valves or switches or may comprise automatic temperature adjusting devices, or may comprise both signalling and automatic temperature adjusting devices.

A general object of the invention is to provide new and novel combinations and arrangements adapted to accomplish the results or purposes above specified.

Another object of the invention is to provide improved signalling and controlling apparatus of I the type referred to above-in which the instrument for measuring the electromotive force variations does not require the use of any physically movable partsdeilecting in accordance withthe variations in the conditions.

Still another object of the invention is to provide improved signalling and controlling apparatus of the type referred to above in which a simplified circuit arrangement for producing the signalling operations is employed.

A more specific object or the inlluntion isgto, provide an improved and simpliiled circi'iiVarrangement for energizing signal lamps t6 produce a visual signal of the deviations in magnitude of a variable condition about a predetermined value.

A further specific object of the invention is to provide an improved instrument for measuring the magnitude and changes in magnitude of the thermocouple electromotive force in signalling and controlling apparatus of the type referred to above in which means are provided for eliminating extraneous fluctuating currents which may be induced in the thermocouple or in the leads extending between the instrument and the thermocouple.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding-of the invention, however, its advantages and specific objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention. I

The single figure of the drawing is a diagrammatic representation of signalling and controlling apparatus embodying my invention.

In the drawing I have illustrated, more or less diagrammatically, a furnace or other compartment to be heated at l in which an electrical heating element 2 is arranged and adapted to be supplied with energizing current from electric current supply conductors 3 and 4. The thermocouple '5 which is sensitively responsive to variations in temperature is mounted in the furnace i and has its terminals connected by conductors 6 and 'I to the terminals of a deflectional potentiometric network 9 which may be located at a point remote from the furnace I.

The potentiometric network 8 may be of any suitable type such as the Brown potentiometer,

circuit disclosed in the Harrison Patent 1,898,124, issued February 21, 1933,. and for the present purposes it is sumcient to note that the potentio- ,metric circuit 8 includes a circuit branch comprising the thermocouple an opposing circuit branch including a source of known potential, such as a battery 8, and a slide wire resistor III a variable portion of which may be connected in the opposed branches by means of a sliding contact whereby the respective eflects of the variable and known sources are made equal and opposite.

Diiferences in the variable and the known sources of electromotive force are adapted to be detected by an electronic device l2 which includes a converter l3 for transforming the unbalanced potentiometri direct currents into an alternating current which may be readily amplified. The converter l3 includes a center tapped primary winding H of a transformer I5 and an interrupter l6 which are connected in the conductor 6 leading to the thermocouple 5. The interrupter I3 operates to periodically interrupt the unbalanced current flow produced in the potentiometric circuit by the diflerential of the variable and known sources of potential to create a pulsating current flow in the transformer primary winding II. This pulsating current flow is translated by the transformer I5 into an alternating current in the secondary winding ll of transformer II. The alternating current so derived in transformer secondary winding I1 is impressed on the input circuit of an amplifier l8, included in electronic device i2, wherein it is amplified, and the amplified quantity is impressed on the input circuit of a reconverter I3, also included in electronic device l2, which operates to reconvert the amplified alternating current to a. unidirectional current. The unidirectional current so derived is impressed on the control winding 20 of a relay 2| to actuate the latter into its open, neutral or closed position.

The converter 3 illustrated schematically in the drawing operates to convert the potentiometric unbalanced direct currents into pulsating currents which are capable of being readily amplified. It will be understood that any suitable current interrupter may be employed for this purpose, and by Way of example may be of the type shown and i1- lustrated in application Serial Number 421,176, which was filed by Frederick W. Side on December 1, 1941, and which issued as Patent No. 2,423,524 on July 8, 1947. The interrupter I6 is essentially a polarized switching mechanism and is provided with an operating winding 22 which is connected to and receives energizing current from the low voltage secondary winding 23 of a combination step-up and step-down transformer 24. The transformer 24 includes a line voltage primary winding 25 which is connected to and receives energizing current from alternating current supply conductors 26 and 21 which may desirably supply 60 cycle alternating current. Transformer 24 also includes high voltage secondary windings 28, 29 and 30 in addition to the low voltage secondary winding 23.

The interrupter l6 also includes a vibrating reed 3| and a pair of relatively stationary contacts 32 and 33 in addition to the operating winding 22. The operating winding 22 is arranged in coperative relation with the reed 3| to vibrate the latter in synchronism with the alternating current sup plied to the winding 22 from the transformer secondary winding 23. Contact 32 is connected to one end terminal of the transformer primary winding l4 and the other end terminal of the latter is connected to the contact 33. The center tap on transformer primary winding I4 is connected to the conductor 3 and the reed 3| of interrupter I6 is connected by a conductor 34 to the contact II in engagement with the potentiometer slide wire resistance Hi. When the vibrating reed 3| is in engagement with the contact 32 the lower half of the primary winding H is connected in circuit with the opposed potentiometric potentials and when the reed 3| is in engagement with the contact 33 the upper half of the primary winding I4 is connected in circuit with those opposed potentials.

For purposes of explanation it may be assumed that. contact 32 of the interrupter I6 is engaged by the vibrating reed 3| during the even half cycles of the alternating current supplied the operating winding 22 and the second contact 33 is engaged by the vibrating reed 3| during the odd half cycles of that alternating current supply. When the reed 3| is in its mid-position both of the contacts 32 and 33 are engaged by the reed 3|, and therefore, when the reed 3| is vibrated it is alternately in engagement with one or the other of the contacts 32 and 33.

When the potentiometric network 8 is balanced, no current fiows in conductors 3 and 34 and consequently operation of the interrupter I6 is ineffective to cause the fiow of current through the primary winding of the transformer l5. When the electromotive force produced by the thermocouple 5 increases to unbalance the potentiometric network 3 in one direction, however, unbalanced potentiometric direct currents fiow through the conductors 6 and 34 in the direction from the interrupter l3 to the transformer primary winding l4, and when the electromotlve force of the thermocouple 5 decreases, unbalanced potentiometric direct currents flow through the conductors 3 and 34 in the direction from the transformer primary winding H to the interrupter l6.

Thus, upon unbalance of the potentiometric network 8 in the directio to render the conductor 6 positive with respect to the conductor 34 unbalanced potentiometric direct currents, during the even half cycles of the alternating current supplied by the transformer secondary winding 23, will fiow from the conductor 6 to the center-tap on the transformer primary winding I4 through the lower half of the primary winding l4 to the interrupter contact 32, and through reed 3| to the conductor 34. During the odd half cycles of the alternating current supply the potentiometric unbalanced current will flow from the conductor 6 through the upper half of the winding 4 to contact 33 and through reed 3| to conductor 34. The pulsating direct current flows through the transformer primary winding |4, first through the lower half and then through the upper half, operate through the core structure of the transformer to induce an alternating voltage in the transformer secondary winding ll of one phase or of opposite phase relatively to the phase of alternating current supplied by the transformer secondary winding 23.

Upon unbalance of the potentlometric network 8 in the opposite direction the conductor 34 will be rendered positive to the conductor 6, and consequently, during the even half cycles of the alternating current supply the potentiometric unbalanced currents will fiow from the conductor 34 to the reed 3|, the contact 32, the lower terminal of the transformer primary winding I4, and through the lower half of the winding H to the conductor 6. During the odd half cycles the unbalanced current will flow from conductor 34 to the reed 3|, the contact 33, the upper terminal of the winding 4, and through the upper half of the winding is to the conductor 8. Again the unbalanced pctentiometric currents fiow first through the lower half and then through the upper half of the winding H, but for this condition of potentiometric unbalance it will be noted that the pulsating direct currents through the opposite halves of the transformer primary winding II are in the opposite direction from what they were when it was unbalanced in the direction first considered, As a result there pulsating direct current flows operate through the core structure of the transformer I! to induce an alternating voltage in the transformer secondary winding 11 which is of the opposite phase relatively to the voltage of the alternating current supplied by th transformer secondary winding 23.

Thus, when the potentiometric network 8 is unbalanced in one direction an alternating voltage of one phase relatively to the voltage supplied by transformer secondary winding 23, and therefore, relative to the voltage supplied by the supply conductors 26 and 21, is produced in the transformer secondary winding l1, and when the potentiometric network is unbalanced in the opposite direction an alternating voltage of the opposite phase relative to the voltage of the supply conductors 26 and 21 is induced in the transformer secondary winding I'I. When the potentiometric network is balanced no current fiows through the primary winding ll of the transformer is and'under this condition the alternating voltage derived in the transformer secondary winding i1 is zero.

The alternating voltage so derived in the transformer secondary winding I1 is amplified by the electronic valve amplifier II. The input stage of the amplifier Ill includes an electronic tube 35 which as shown is of the double triode type. One triode of the valve 35 has been designated by the reference numeral 38 and the-other triode thereof has been designated by the reference numeral 31. Both triodes l8 and 31 include anode control electrode, cathode and heater filament elements which are all contained within one envelope. The heater filaments of the triodes are connected in parallel and receive energizing current from the low voltage secondary winding 23 of the transformer 2| through conductors whlch'have not been shown in order to avoid complication of the drawing. The transformer secondary winding 23 is also connected by means of conductors not shown to the heater filaments of the. electronic valve 38 also included in the amplifier l8. The valve Ill includes two heater type triodes designated by the reference character 39 and 40 within the same envelope. Both of the triodes of valve 38 include anode, control electrode, cathode and heater filament elements. g

The triode 40 of the valve 38 is utilized as a half wave rectifier to provide a source of unidirectional voltage to energize the output circuits of the triodes 36, 31 and 39. The control electrode and cathode of triode 40 are connected to each other and the anode circuit of triode 40 is energized by the transformer secondary winding 28 through a circuit which may be traced from the right end terminal of the winding 28 as seen in the drawing through conductor ll to the anode of triode 40, the cathode thereof and through a conductor 42 to the positive terminal 43 of a filter generally designated by the reference character'. The negative terminal 45 of the filter is connected by a conductor 46 to the left end terminal of the transformer secondary winding 28. The filter 44 includes a condenser 41 which operates to smooth out the ripple in the output voltage of the filter between the points 48 and ll. The filter 44 also includes a resistance 08 and a condenser ll which operate to smooth out the ripple between the filter points ill and 45. Filter 44 also includes a resistance 5i and a condenser 52 for smoothing out the output voltage between the points II and 45. Thus, the filter comprises 8 stages. Such a three stage filter is provided because for satisfactory and eflicient operation it is desirable for the anode voltage supplied to the triode 30 to be substantially free from ripple whereas it is not necessary to supply anode voltage so free from ippie to the output circuit of triode 31. In "addition, it is not necessary to supply anode voltage as completely free from ripple to the triode 3! as it is to the triode 31.

The anode circuit of the triode 38 may be traced from the filter point I! which comprises a positive terminal of the filter through a fixed resistance 54 to the anode of the triode 3!, the anode to cathode resistance, and through a cathode biasing resistance 55 which is shunted by a condenser 56 to the negative filter point ll through a conductor 51. The cathode biasing resistance 55 and the parallel connected condenser 86 are utilized for biasing the control electrode of triode I6 negatively with respect to the cathode. The input circuit of the triode 36 ma be traced from the cathode through the parallel connected resistance 58 and condenser I6 through the transformer secondary winding II. to the control electrode of the triode 38. A resistance 58 is provided in shunt to the transformer secondary winding IT for tuning the latter. I! desired, a condenser may be connected in shunt to the transformer secondary winding I! for this purpose.

The output circuit of the triode 86 is resistance capacity coupled to the input circuit of triode 31 by means of a condenser I! and a resistance Ill. To this end the anode of the triode 38 is connected by the condenser is to the cohtrolelectrode of the triode l1 and the control electrode of triode I1 is connected by the resistance 80 to the conductor I! and thereby the cathode of triode 31. The anode circuit of triode 81 maybe traced from the positive terminal II of the filter 44 through a fixed resistance CI to the anode of triode 31, the cathode thereof and conductor 81 to the negative terminal 45 ofthe filter. The output circuit of the triode 31 is resistance capacity coupled by means of a condenser 62 and a resistance 63 to the input circuit of the triode 39. As shown, the control electrode of triode 38 is connected to a contact 84 which is adjustable along the length of resistance It.

The resistances O0 and 63 which are connected in the input circuits of the triodes 31 and 39,

respectively, operate to maintain the potentials of the control electrodes of triodes l1 and-39 at the same potentials as their associated cathodes. Upon the induction of a voltage in the transformer secondary winding II the resistances 60 and 63 permit the flow of grid current between the control electrodes of the triodes 31 and 39 and their associated cathodes and thereby limit the extent the control electrodes of the triodes are permitted to go positive with respect to their associated cathodes. The contact 84 which is provided in adjustable engagement with the resistance 63 is employed for varying the proportion of signal from the outputcircuit of the triode 31 which is impressed on the input circuit of the triode 3s and thereby for varying the sensitivity of the electronic amplifier ll. 4

The anode circuit or the triode 39 may be traced from the positive terminals 49 of the filter 44 through a fixed resistance 69 to the anode of triode 99, the anode to cathode resistance, and

through conductor 61 to the negative terminal ll of the filter.

The output circuit of the triode 99 is resistance capacity coupled by means of a condenser 96 and a resistance 6'! to the input circuit of an electronic valve 69. The electronic valve 99 includes two triodes 69 and 19 within the same envelope each of which has anode, control elec trode, cathode and heater filament elements. One triode of valve 69 has been designated by reference numeral 69 and the other by the reference numeral 19. The heater filaments of triodes 69 and 19 may desirably be connected in parallel with the heater filaments of valves 96 and 99 to the transformer secondary winding 29.

The input circuit of valve 99 comprises the input circuits of the triodes 69 and 19 which are connected in parallel and are coupled to the output circuit of the triode 99. To this end the anode of the triode 99 is connected by condensers 96 to the parallel connected control electrodes of triodes 69 and 19, and the control electrodes are connected by resistance 61 to the cathodes of triodes 69 and I9.

Anode voltage is supplied the output circuits of the triodes 69 and 19 from the high voltage secondary windings 29 and 39, respectively, of the transformer 24. That is to say, the anode of the triode 69 is connected to the right end terminal or the transformer secondary winding 29 and the anode of the triode I9 is connected to the left end terminal of the transformer secondary winding 39. The left end terminal of the transformer secondary winding 29 is connected by a conductor 1|, in which a resistance 12 shunted by a condenser 13 is inserted, to the cathode of triode 69 and the right end terminal of the transformer secondary winding 99 is connected by conductor 14, in which a resistance 15 shunted by a condenser 16 is inserted, to the cathode oi triode 19'. As shown, the cathodes of the triodes 69 and 19 are connected to each other and to the point of engagement of resistances l2 and 15.

Thus, the anode circuit of the triode 69 is completed through the resistance 12 and the flow oi anode current through this resistance operates to render the lower end of the resistance 12 positive with respect to the upper end. The anode circuit of the triode I9, on the other hand, is completed through the resistance 15 and the fiow of anode current through this resistance operates to render the upper end of resistance 15 positive with respect to the lower end. The transformer secondary windings 29 and 39 are so wound on the transformer 24 that the triodes 69 and 19 are arranged to be rendered conductive during alternate half cycles. Condensers l3 and 16 are provided to maintain the potential drops across resistances l2 and 15 during the half cycles that the associated triodes 69 and 19 are non-cnductive.

The output circuits of the triodes 69 and 19 are coupled to the input circuit of an electronic valve 11. The valve 11 may be of the type known and sold commercially as beam-power amplifier valves and is provided with anode, screen, control electrode, cathode, heater filament, and beam forming electrodes, and is arranged to be supplied with anode voltage from the transformer secondary winding 39. The heater filament of valve II is connected by conductors, not shown, to the low voltage transformer secondary winding 29 in parallel with the heater filaments of valves 99, 99 and 99.

The input circuit of the valve 11 may be traced from the control electrode through a grid current limiting resistance I9 and through the resistances l2 and 16 to the cathode. It will be noted that the potential drop across the resistance 12 tends to drive the control electrode of the valve 11 negative with respect to the potential of its associated cathode and that the potential drop across resistance 16 tends to drive the control electrode positive with respect to the potential of its associated cathode. Accordingly, when the triodes 69 and II are equally conductive the potential of the control electrode of valve 11 will be the same as that oi its cathode. when the triode 99 becomes more conductive than the triode 19, however, the control electrode or the valve 11 will be driven negative while it will tend to be driven positive when the triode 19 becomes more conductive than the triode 69.

The output circuit of the valve 11 may be traced from the left end terminal or thetransi'ormer secondary winding 99 to a conductor 19, in which the operating coil 29 of relay 2| is inserted, to the anode of valve 11 and from the cathode thereof to the conductor 14 to the right end terminal of the winding 99. A condenser 99 is provided in shunt to the operating coil of relay 2| for smoothing the ripple in the output current from the valve H which flows through the winding 29. This operates to prevent chattering oi the relay.

The current fiow in the output circuit oi the valve I1 and thereby through the operating coil of 29 of relay 2| is a predetermined value when the triodes 69 and 19 are equally conductive and is increased when a signal is impressed upon the input circuit or the triodes 99 and I9 oi the proper phase to render the triode 19 more conductive than the triode 69. On the other hand the flow in the output circuit and through the coil is decreased when the signal impressed on the input circuit of the triodes 69 and I9 is of the proper phase to render the triode .69 more conductive than the triode 19.

The relay 2| includes an armature 9| which is pivoted for rotation at a point 92 and is arranged in cooperative relation with a pair of relatively stationary contacts 93 and 94. The armature 9| is biased by gravity or suitable spring means {or rotation in n counterclockwise direction about its pivot point 92 and is actuated by the operating coil 29 of relay 2| for rotation in a clockwise direction. When the potentiometric network 9 is precisely balanced the current flow through the operating coil 29 is the proper value to actuate the armature 9| to a position in which it is out of engagement with both of the contacts 93 and 94. When the potentiometric network 9 is unbalanced in the direction to increase the conductivity of the triode 19 relative to that of the triode 69 and thereby to increase the current in the operating coil 29, the armature 9| is actuated into engagement with the contact 93. When the potentiometric network 9 is unbalanced in the opposite direction the current flow through the operating coil 29 is decreased and the armature 9| is then actuated by gravity or its associated spring means into engagement with the contact 94. Thus, if the temperature of the furnace rises above or falls below the predetermined value at which the apparatus is desig- 9 natcd to maintain the temp rature, the armature II is actuated into engagement with one or the other of its associated contacts 88 and 88.

In accordance with the present invention the deflections of the armature 8| into engagement with the contacts 88 and 88 from the position in which it does not engage either contact are utilized to simultaneously selectively energize one of a plurality of electric lamps 88, 88 and 81 to provide a visual indication or signal of the temperature condition within the furnace I. To this end the armature 8| is connected by a conductor to one terminal of a source of alternating current which may comprise a secondary Winding provided on the transformer 24 but which has not been shown in order to avoid complication of the drawing, and the other terminal of that alternating current source is connected by a conductor, in which a resistance 88 is inserted, to a common terminal of the lamps 88, 88 and 81. The other terminal of the lamp 88 is connected to contact 83 of relay 2| and the other terminal of lamp 88 is connected to armature 8|. The other terminal of lamp 81 is connected to the contact 84 of relay 2|. By way of illustration it is noted that when the voltage of the alternating current source which is utilized for energizing lamps 85, 88 and 81 is 6.3 volts, the resistance 88 may have a value of 7.5 ohms and the lamps 88 and 81 may be 2.5 volt lamps and the lamp 88 may be a 6.3 volt lamp.

When the relay armature 8I is intermediate and out of engagement with both of the contacts 83 and 84 only the lamp 88 will be connected in circuit through the resistance 88 to the terminals of the alternating current supply source. Since I I 1o the lamp 88, the latter is eflectively deenergize for illumination but the voltage impressed on the lamps is high enough to illuminate the lamp 88. Consequently, the lamp 88 is then illuminated and provides a visual indication or signal that the temperature within the furnace I is below the the lamp 88 is a 6.3 volt lamp and the voltage of the alternating current source is 6.3 volts, the lamp 88 will then be illuminated to visually indicate that the potentiometric network 8 is precisely balanced and that the temperature within the furnace I is at the desired, normal value.

Upon a change in the furnace temperature, for example, upon an increase in temperature the potentiometric network 8 will be unbalanced in the direction to effect an increase in the conductivity of triode 88 relative to that of triode 18 and thereby to effect a decrease in the current through the operating coil 28 of the relay 2 I The armature 8| will then be actuated into engagement with the contact 88 and thus will operate to connect the lamp 81 in shunt to the lamp 88 across the terminals of the alternating current supply source. The connection of lamp 81 in parallel to the lamp 88 will operate toeffectively deenergize the lamp 88 for illumination because the filament of lamp 81 is of lower resistance and is designed to be illuminated with only 2.5 volts impressed thereon. Thus, the connection of lamp 81 in parallel with lamp 88 operates to reduce the voltage impressed across both of the lamps to a point below the voltage at which the lamp 88 is illuminated. The voltage impressed on the lamps 88 and 81, is not reduced below the point at which the lamp 81 is illuminated, however. The illumination of lamp 81 provides a visual indication of the increase in temperature within the furnace I.

Upon a decrease in the furnace temperature the potentiometric network 8 is unbalanced in the opposite direction to effect an increase in the current flow through the operating coil of relay 2| and thereby actuation of the relay armature 8| into engagement with contact 88. The lamp 85 is then connected in parallel with the lamp 88 and because of the reduced voltage impressed on control point.

The relay 2| is also provided with a second armature 88 which is biased by gravity or by suitable spring means (not shown) in a counter clockwise direction-and is adapted to be rotated in a clockwise direction into engagement with a contact 88 upon energization of the operating coil 28 of the relay. The armature 88 and contact 88 are connected in the energizing circuit to the furnace heating resistance 2 from the electric current supply conductors 8 and l. The armature 88 is so arranged with respect to the contact 88 that when the temperature within the furnace I is at the desired value and the potentiometric network 8 is precisely balanced the armature 88 is maintained out of engagement with the contact 88. Upon a slight decrease of the temperature within the furnace I from the desired value the potentiometric network 8 is unbalanced in the proper direction to effect an increase in the energizing current flow through the operating coil of relay 2| to eifect actuation of the armature 88 into engagement with the contact 88, and thereby to close the energizing circuit to the furnace heating resistance 2 from the electric current supply conductors 8 and 8. Upon increase in the temperature within the furnace I from the desired value the potentiometric network 8 is unbalanced in the opposite direction to effect a decrease in the energizing current flow through the operating coil 28 of relay 2| and thereby actuation of the contact 88 away from the contact 88.

The temperature value which thecontrol apparatus described operates to maintain within the furnace I may be adjusted by varying the position of contact II along the length of the slidewire resistance I8. To facilitate such adjustment a scale calibrated in terms of temperature maybe arranged in cooperative relation with the con- 7 tact and the contact II may be adjusted by manipulation of a knob, as shown.

In order to eliminate the effects of undesired I stray alternating 0r fluctuating currents which may be extraneously induced into the potentiometric network 8 or in the thermocouple 8 or in its conducting leads 8, and '1, a condenser I88 is provided between the center-tap on the transformer primary winding I8 and the ground conductor 81. As shown, the core structure of the transformer I5 is also connected to the ground conductor 81. I have discovered that by providingv condenser I88 connected as shown, any stray alternating or fluctuating currents which may be induced in the thermocouple leads are by-passed to ground and their effect on the operation of the measuring and control apparatus is thus mate,- rially reduced if not altogether eliminated.

While in accordance with the provisions of the statutes, I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims and that in some cases certain features of my invention may be used to advantage without a corresponding use of other features.

Having now described my invention, what I 11 claim as new and desire to secure by Letters Patent is:

1. Signalling means including th'ree electrical signalling devices, one or said devices operating with a diflerent impressed voltage than the other two devices, an electric circuit including said one device and a source or electric current, and means for selectively shunting said one device with one or the other of the other 'two devices.

2. signalling means including three electrical signalling devices, one of said devices operating with a predetermined impressed voltage thereon which is higher than the impressed voltage required to energize the oth'er two devices, an electric circuit including said one device and a source or electric current, and means for selectively shunting said one device with one or the other of the other two devices.

3. Signalling means including three electric lamps, one of said lamps being adapted to be illuminated with a predetermined impressed voltage thereon which is higher than the impressed voltage required to illuminate the other two lamps, an electric circuit including said one lamp and a source 01' electric current, and means for selectively shunting said one lamp with one or the other of the other two lamps.

4. Means for lighting and/or extinguishing one of three signal lamps including a pair of terminals adapted for connection to an energizing voltage source, one of said lamps operating with a prede- 12 termined impressed voltage thereon which is higher than the impressed voltage required to energize the other two lamps. a resistance permsnently connected in series with said one of said lamps between said terminals, and means for selectively shunting said one lamp with one or theother of the other two lamps.

5. Means for selectively actuating one of three electrical signalling devices including a pair of terminals adapted for connection to an energising voltage source, one or said devices operating with a predetermined impressed voltage thereon which is higher than the impressed voltage required to energize the other two devices, an impedance permanently connected in series with said one device between said terminals and means tor selectively shunting said one device with one or the other of the other two devices.

HARRY 8. JONES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,443,165 Brown Jan. 23, 1923 1,443,166 Brown Jan. 23, 1923 1,496,875 Field June 10, 1924 1,909,712 Oppegaard May 16, 1933 

